South American Leaf Blight (SALB) field resistance to the fungus Microcyclus ulei (P. Henn.) v. Arx was observed on an ex-situ germplasm collection of rubber trees [Hevea brasiliensis (Willd. ex Adr. De Juss.) Muell.-Arg.], planted both in French Guyana and in Brazil, wich included 298 accessions from a survey in the Brazilian states of Acre, Mato Grosso and Rondônia. Two sets of observations were made in French Guyana, in early 1999 and Iate 2000, but only one set was made in Brazil in early 1999. Results showed a high ratio of SALB-susceptible clones among Mato Grosso origins (up to 81%), whereas clones from Acre or Rondônia were the most resistant . The resistance of the Mato Grosso clones seems to be more unstable than that found in of the Acre or Rondônia clones. This result is consistent with the data reported on genetic diversity of H. brasiliensis, and can explain the lack of resistance to M. ulei found in the Asiatic clones.